Frequency control circuits



Julyv 22, 1941.

Sl/VE WH VE K. la.v WENDT 2,250,284

FREQUENCY CONTROL 4CIRCUITS Filed oct. 26, 193s 2 sheets-sheet 1 F26. FefQU/SA/cy D/wofe.:

OSC/LLHTOH FREQUENCY Co/vreol.

N64/VJ F04? OJC/m me i FG.3.

Snventor Wendt July 22, 1941. K. R. wENDT FREQUENCY CONTROL CIRCUITS Filed OCT.. 26, 1938 2 SheetS-S'heei 2 i Snvcntor Karl R. We 71, d

Patented July 22, 1941 FREQUENCY CoN'raoL CIRCUITS Karl R. Wendt, Audubon, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application october ze, 193s, serial No. 237,051

(Ci. o-3s) 9 Claims.

My invention relates to the frequency or phase control of oscillators and particularly to means for locking in an oscillator with' a standard frequency source.

The invention will be described with reference to a television transmitter where it is desired to lock in a synchronizing impulse generator with the power line frequency. A system of this general characterv is described in Patent No.

2,137,010, issued November 15, 1938, in the name` of A. V. Bedford and assigned to the Radio Corporation of America.

It is an object of my invention to provide an improved frequency control or lock-in circuit for oscillators.

It is a'further object of my invention to provide an improved circuit for indicating the phase difference between electrical waves or signals.

A still further object of my invention is to provide an improved means for locking in a synchronizing impulse generator or a cathode ray tube deecting circuit with a source of vcontrolling frequency such as a power line.

In practicing my invention I charge a condenser across which the controlling voltage is to appear in accordance with the phase relation of the control source with respect to electrical impulses derived from the oscillator being controlled. The condenser is prevented from discharging between the occurrence of successive impulses by means of a rectifier in the charging circuit. p Additional means is provided to permit a discharge rather than a charge of the condenser upon the occurrence of an electrical vimpulse if the phase relations are such as to require this for bringing the oscillator back to the desired phase relation. A control circuit operating on this principle is described and claimed in Patent No. 2,201,978, issued May 28, 1940, in the name of A. V. Bedford, entitled Frequency control circuits, and assigned to Radio Corporation of America.

My circuit for obtaining this result differs from the Bedford circuit, however, and has certain advantages. In its preferred form it is a balanced bridge circuit comprising four diodes, the

CII

60 cycle control voltage being impressed across charged through two of the diodes or discharged through the other two diodes, depending upon which side of the equilibrium point the phase has shifted.

'I'he invention will be better understood by referring to the accompanying drawing in which Figure 1 is a circuit diagram of a preferred embodiment of my invention,

Figures 2 to 4 are curves which are referred to in explaining the invention. and

Figures 5 and 6 are circuit diagrams of other embodiments of my invention.

Referring to Fig. 1, I have shown my invention applied to a television transmitter which includes a synchronizing impulse generator comprising an oscillator I0. an automatic frequency control or A. F. C. circuit II for the oscillator I0, and a. group of frequency dividers I2, I3, il and I6. The oscillator IIJ may be a conventional `sine wave oscillator while the A. F. C. circuit may be of any suitable design as indicated in the second-mentioned Bedford Patent 2,201,978. 'Ihe several frequency dividers may bemultivibratcrs.

Referring to the control circuit, it comprises four rectiers such as diodes I1, I8, I9 and 2| connected to form a bridge. It will be noted that the plates of diodes I'I and I8 are connected together, that the cathodes of diodes I9 and 2| are connected together, and that the bridge circuit is completed by connecting the cathode of diode I8 to the plate of diode I9 and the plate of diode 2| to the cathode of diode I1.

The cycle supply from the power line is connected across one diagonal of the bridge through a control condenser 22, the power line output being connected between ground and the junction point of the cathode and plate of diodes I1 and 2|, respectively, and the condenser 22 being connected between ground and the junction point of the cathode and plate of Vdiodes I8 and I9, respectively.

Impulses from the frequencydivider I6 are impressed across the other diagonal of the bridge through a transformer 23. The` secondary 24 of this transformer is connected in series with an RC network comprising 'a resistor 26 shunted by a condenser 21, whereby there is builtup across the condenser 21 a voltage which opposes the voltage across the secondary 24. The values indicated on the drawing for the RC network are merely by way of example.

It should be noted that the impulses supplied through transformer 2l across the one bridge diagonal never appear across the other diagonal since the bridge is balanced.

The'voltage across the condenser 221s applied to the A. F. C. circuit II whereby the frequency oi the oscillator III is corrected in accordance with changes in the condenser voltage.

The way in which my circuit operates to produce the desired control voltage will now be described. 'I'he electrical impulses supplied from the frequency divider I6 to the' transformer 23 are shown at a in Fig. 2. These impulses a preferably are substantially rectangular so that they have a steep side which produces a sharp impulse b (Fig. 3) across the secondary 24.

The sharp impulses b are obtained since the transformer 23 is designed to pass only thehigh frequency components of the impulses a. The impulses a may be of any convenient width, the impulses b' of reverse polarity (Fig. 3) not being made use or'.

The 60 cycle voltage wave supplied to the bridgefrom thepower line is indicated at c in Fig. 4.v In the example `being described, the wave c' is substantially a sine wave but it may have a different shape. ,Preferably however, thesides of the wave c are sloping.

The impulses b appear across secondary 24 with a polarity which drives the plates of diodes vdiodes are electrically conducting at this instant whereby the condenser 21 of the RC network is charged through diodes I8 and I9 in series and through diodes I1 and 2| in series. As the time constant of the RC circuit is large as compared to the time between successive impulses b, a voltage builds up across condenser 21 and resistor 26 whichis opposing and nearly equal to the voltageV impulses b.

Now assume, for example, that the positive half of the wave c occurs at the same time as the impulse b. For the duration oi. the impulse b, all diodes are conducting. Thus the power-line source charges the condenser 22 at this instant through the diode 2|, the secondary 24, the RC network 25, 21 and the diode I8. It will be seen that the charge will depend upon the instantaneous value of wave c at the instant the impulse b occurs, and that this instantaneous charging value depends upon the phase relation ofimpulse b and the wave c.

If, at the instant van impulse b occurs, the instantaneous voltage of wave c is the same as the voltage across condenser 22, there will be no charge or discharge of condenser` 22. If the instantaneousvoltage of wave c is greater than the voltage across condenser 22 it will receive an additional charge as described above.

If the instantaneous voltage of wave c is less than the voltage across condenser 22 at the time an impulse b occurs, the condenser 22 will discharge a slight `amount to lower its voltage to the new instantaneous value, this discharge being through diode I9, thesecondary 24, the RC network and the diode I1.

Between impulses b, the condenser 22 retains its Vcharge since it cannot discharge through the diodes I9 and I1 because of the opposing voltage across the RC network 26, 21. It will be From the foregoing description it will be apparent that the condenser 22 is provided with both a charging circuit and a dischargingcircuit, and that means is provided to prevent the condenser from discharging between control impulses.

The above-,described circuit is a preferred embodiment oi' my invention because it is always balanced if the diodes are similar as they preferably are. c,

In Fig. there is shown a circuit which is satisfactory although it is sometimes diiiicult to balance. It comprises two diodes 3| and 32 with the cathode of one diode connected to the plate of the otherV diode. The control impulses b are impressed yacross the ydiodes 3|v and 32 through a transformer 33 just as in Fig. 1.

similar networks are provided, one comprising a resistor 34 and a condenser 36 between the transformer 33 and the diode 3| and the other comprising a resistor 31 and a condenser 38 between the transformer 33 and the diode 32.

The 60 cycle power linevoltage c is applied between ground and the junction point of the cathode and plate of diodes 3| and 32, respectively.

The condenser 39 across lwhich appears the control voltage or the phase indicating voltage is connected between ground and the midpoint of the secondary o'f transformer 33. In operation, condenser 39 is charged through diode 32 and the RC network 38, 31 if the instantaneous sine wave voltage is greater than the voltage of condenser 39 'at the instant the sharp impulse occurs. If the instantaneous sine wave voltage is lower at this time than the voltage of condenser 39, condenser 39 dischargesr noted that in order for the voltage across the e Discharge of condenser 39 between the occurrence yoi? the sharp voltage impulses is prevented by the voltage built up across the RC network 34, 36 as in the case of the circuit in Fig. 1. The RC network 31, 38, which is the same as RC network 34, 36, prevents charging of condenser 39 between the occurrence of successive sharp voltage impulses. It will be seen that the circuit is balanced whereby none oi the sharp voltage impulses b appears across the condenser 39. k In' Fig. 6 there is shown a circuit which is similar to the circuit of Fig. 5 but which is easier to balance. In the two figures like parts are indicated by the same reference numerals. In Fig. 6 a single" RC network is employed comprising a resistor 4I shunted by like condensers 42 and 43, this network being located between the cathode and plate of diodes 3| and 32, respectively. v

The 60 cycle supply is applied to the control circuit through a variable tap 45 on resistor 4| whereby, except possibly for the presence of undesired stray capacities, the circuit may be properly balanced.

The operation of the circuit of Fig. 6 is substant5ially 'the same as that of the circuit in Fig.

It may be noted that the impulses a and b need not occur at the same frequency as the control wave c but they should have a harmonc relation thereto. For example, assuming wave dto have a frequency of 60 cycles per second, impulses a may occur at the rate of 60 per second, 30 per second, or 20 per second.

It will be understood that the use of the transformer 23 may be avoided if -the impulses a are Twonc" made narrow enough to be applied directly across the bridge diagonal. In general, however, it is advantageous to use a transformer both because the width of the impulse a then need not be adjusted to any specific value and becauseit con-v veniently permits operating the diodes above ground potential. i

As indicated in the foregoing description, my balanced control circuits preferably include RC circuits across which there automatically builds up a biasing or opposing voltage which is slightlyvless than the amplitude of -the sharp impulses b. However, batteries or other biasing sources may be substituted for the RC networks, the battery voltage being adjusted to a value slightly less than the amplitude of the impulses b, and satisfactory results will be obtained so long as the lmpulses b remain substantially constant in ampli-tude.

I claim as my invention:

1. A circuit for producing a voltage representative'of the phase relation between the output of a iirst signal source and the output of a second signal source, said outputs having a harmonic relation to each other, said circuit comprising a condenser across which said voltage is to appear, a charging circuit including a rectier, a discharging circuit including a rectifier, said sec' ond source being connected in said charging and discharging circuits to charge said condenser through said charging circuit, means for deriving from said rst source voltage impulses of greater amplitude than the output of said second source, means for applying said impulses in a bridging connection to said charging and discharging circuits whereby said impulses are applied thereto in balanced relation to the connections of said second source in said charging and discharging circuits and for making said charging circuit effective to pass a charging current only in response to and for the duration of each of said voltage impulsesand for simultaneously making said discharging circuit eiective to discharge said condenser only during said voltage impulses and in response to the charging voltage being less than the voltage across said condenser.

2. A circuit for producing a voltage representative of the phase relation between the output of e. rst signal source and the output of a second signal source, said outputs liaving a harmonic relation to each other, said circuit comprising a condenser across which said voltage is toappear, saidy second source being connected to charge said condenser through a charging circuit, said charging circuit including means for providing a biasing voltage and including rectifier means in series therewith, said biasing voltage being poled to oppose current iiow through said rectifier, a discharging circuit'including a rectiiier, means for deriving from said i'lrst source voltage impulses of greater amplitude than the output of said second source, means including said biasing source for making said charging circuit effective to pass a charging current only in response to and for the duration of each of said voltage impulses, and means for simultaneously making said discharging circuit effective to discharge said condenser only during said voltage impulses and in response to the charging voltage being less than the voltage across said condenser.

3. A circuit for producing a voltage representative of the phase relation between the output of a first signal source and the output of a second signal source, a condenser across which voltage is to appear, a pair of rectiflers each having a cathode and an anode, the cathode of one rectiiier being connected to the anode of the other rectifier whereby there is a junction point, means for supplying voltage impulses of a certain polarity from said first source acrosssaid rectiers in series, saidimpulses being of short duration and of greater amplitude than the instantaneous voltage of the output of the second source, a source of biasing voltage of slightly less amplitude than said impulses connected in series with said rectilers and said impulse supplying means with the biasing voltage in opposition to the Voltage impulses of said certain polarity, and means for connecting said second source and said condenserl in series between said junction point of one cathode and one anode and the circuit connecting the other cathode and anode of said rectiiiers and in balanced relation with respect to said lrst source whereby said impulses do not appear acrosssaid condenser.

4. A circuit for producing a voltage representative of the phase relation between the output of a iirst signal source and the output of a second signal source, a condenser across which said voltage is to appear, a, rectifier, means for charging said condenser from said second source through a charging circuit including said rectier when said'phasel relation is on one side of an equilibrium point, a second rectiiier, means for partially discharging said condenser through said second rectifier in response to the occurrence of voltage impulses derived from said iirst source when said phase relation is on the other side of said equilibrium point, and means for preventing said condenser from either charging or discharging between successive voltage impulses, said last means comprising a resistor-capacitor circuit, said condenser, said second rectiiiensaid last means and said second source comprising a closed series circuit, said resistor-capacitor circuit comprising a resistor shunted by a condenser.

5. A circuit for controlling the frequency or phase of an oscillator in accordance with the frequency of a controlling source, said circuit comprising a pair of rectiiiers each having a 4cathode and a plate with the plates being didiodes whereby a bridge circuit is formed, a condenser across which a controlling voltage is to appear, means for supplying voltage from said controll-ing source across one diagonal of said bridge through said condenser, the terminals of said one diagonal being the junction points of the cathodes of said first pair of diodes and the plates of said second pair of diodes, means for impressing across the other diagonal of said bridge periodic sharp voltage impulses of a certain polarity derived from said oscillator and having a harmonic relation to the frequency of the controlling source, said voltage impulses having a greater amplitude than the peak voltage supplied by the controlling source. and a resistor-capacitor circuit connected in series in said other diagonal and having a time constant such that there is built up thereacross a substantially i steady voltage which opposes the voltage impulses of said certain polarity.

6. vA circuit for producing a voltage representative of the phase relation between a lirst signal lsource and a second signal source, said circuit comprising a pair of rectiflers each having a cathode and an anode, the cathode of one rectifier being connected to the anode of the other rectiiler whereby there is a Junction point of said cathode and said anode, agtransformer having a primary and a secondary, the remaining cathode and anode of said rectifiers being connected to each other through said secondary, a condenser across which said representative voltage is to appear, means for impressing the signal from said second source between said junction point and the mid-pointof said secondary throu'gh said condenser, means for impressing the signal from said first source across said primary to provduce sharp voltage pulses across the secondary, and means connected in series with said secondary and said rectiiiers for preventing discharge of said condenser between successive impulses appearing across said secondary, said last means comprising a resistor shunted by a condenser.

7. A circuit for producing a voltage representative of the phase relation between the output of a ilrst signal source and the output of a second signal source, a condenser across which said yoltage is to appear. four rectiers connected in the form of a bridge or ring with two adjacent rectiiiers connected to conduct in one direction around the ring and the other two rectiers connected to conduct in the opposite direction around the ring, a biasing source, means for deriving sharp voltage impulses of Aa certain polarity from said rst source and supplying them through said biasing source across (v the bridge diagonal having as its terminals the junction points connecting said two adjacent rectiflers in series to said other two rectiilers in series, the voltage acrossv said biasing source being in opposition to and slightly less than the voltage of said impulses of said certain polarity, and means for` supplying the signal from said second source across the other diagonal of said bridge in series with said condenser, the instantaneous voltage of the signal from said second source being less than the amplitude of said voltage impulses.

8. The invention according to claim 7 characterized in that said biasing source comprises a v resistor having a condenser in parallel therewith to form a resistor-capacitor circuit, the time constant of said resistor-capacitor circuit being large compared with the time between the occurrence of said sharp voltage impulses.

9. The invention according to claim '7 characterized in that said means for supplying sharp KARL R. WENDT. 

